It is well known that control circuits including such elements as switches, motors, transducers, and other devices having interrelated functions, operational sequences, and so forth may often be implemented using relay devices. Simply defined, a relay is a device having a control element, typically in the form of a coil, and a controlled element, typically in the form of an armature and a set of contacts. The state of the contacts, either opened or closed, is dependent upon the so-called "normal" condition of the contacts and the presence or absence of current through the coil. Accordingly, it is possible to accomplish a wide variety of control functions, operating sequences, and so forth using relays in combinations of various complexity.
The implementation of a relay circuit is in itself often a difficult step in that the physical location of components often rises to complex and confusing patterns of wires, cables, and other forms of conductors between such elements. Accordingly, difficulty arises in terms of space conservation and also in terms of troubleshooting operations and repair functions. The understandability of relay circuits has been greatly improved by use of so-called "ladder diagrams". This is essentially a device for systematizing the relay-type control circuit using drafting conventions so as to aid the designer and others who are involved with the operation of a control circuit to understand the wiring arrangement and the functional relationships between parts. Ladder diagrams do not, however, eliminate the basic problems in the actual implementation of relay-type control circuits using conventional relay devices.
Control circuits may also be implemented using solid state logic devices, such as gates, flip-flops, and inverters, thus to eliminate most of the space conservation problem associated with devices, such as solenoid-armature type relays. Small, low-power, solid-state devices are also often more easily interconnected than larger hardwired relay devices. Logic circuits, however, require a high degree of technical expertise for functional understanding including, for example, a mastery of Boolean arithmetic principles. In addition, logic circuitry is only utilized to greatest advantage when it is reduced to circuit board state, a step which typically freezes the circuit design and eliminates the possibility for substantial expansion or modification of the control circuit.